Chaetozone shackletoni, Blake, 2018

Blake, James A., 2018, Bitentaculate Cirratulidae (Annelida, Polychaeta) collected chiefly during cruises of the R / V Anton Bruun, USNS Eltanin, USCG Glacier, R / V Hero, RVIB Nathaniel B. Palmer, and R / V Polarstern from the Southern Ocean, Antarctica, and off Western South America, Zootaxa 4537 (1), pp. 1-130 : 96-98

Chaetozone shackletoni new species

Figures 51 B–C, 52, 53

Material Examined. Scotia Sea, Powell Basin, R / V Polarstern, ANDEEP III (ANT-XXII/3), Sta. PS67/121-10, 14 Mar 2002, 63°41.74ʹS, 50°42.99ʹW, box corer, 2621 m, holotype ( SMF 24934) and 2 paratypes with fragments ( SMF 24935).— Drake Passage , off South Shetland Islands , R / V Polarstern, ANDEEP I (ANT-XIX/3), Sta. PS61/105-5, 14 Feb 2002, 61°23.72ʹS, 58°50.04ʹW, box corer, 2266 m, 5 paratypes, ( SMF 24936).— Weddell Sea off Cape Norvegia , R / V Polarstern, ANDEEP III (ANT-XXI/3), Sta. PS67/78-6, 21 Feb 2005, 71°09.45ʹS, 14°0.32ʹW, box corer, 2168 m, 1 paratype on SEM stub ( MCZ149853 ); off Coats Land , USCG Glacier, S ta. 69- 19, 11 Mar 1969, 74.105°S, 32.605°W, 1622 m, 1 paratype ( USNM 1490775 ); Sta. 69-20, 12 Mar 1969, 73.823°S, 31.682°W, 2288 m, 2 paratypes ( USNM 1490776 ). Detached posterior ends present. Some specimens photographed alive during ANDEEP I cruise complete (Sta. PS 67/121-10) but fragmented during preservation .

Description. A small to moderately-sized species; holotype mostly complete, 10.9 mm long, 0.35 mm wide across anterior setigers, 0.8 mm wide across expanded middle segments, with about 50 setigerous segments; incomplete paratype from Glacier Sta. 69-19, 9.5 mm long, 0.4 mm wide across anterior setigers, 1 mm wide across expanded middle segments, with 43 segments. Body with distinctive shape, including narrow anterior thoracic region of about 15–20 setigers, enlarged middle body “stomach” area, and narrow posterior setigers ( Fig. 52A, C ). Anterior and middle segments generally cylindrical in cross section, with narrow, shallow mid-dorsal groove present ( Fig. 53A ) or absent in anterior thoracic segments; venter of thoracic segments weakly flattened, or with shallow groove present or absent; posterior segments somewhat dorsoventrally flattened; weakly developed. Expanded middle segments prominent, with enlarged stomach area filled with fine sediment ( Fig. 52A, C ), also swollen with eggs in mature specimens. Color in life: deep green, darkest in posterior setigers; color in alcohol light tan, with distinctive brown to reddish pigmentation throughout body.

Prostomium short, conical, tapering to narrow rounded tip ( Figs. 51B , 52A , C–D, 53A–C); eyespots absent; circular ciliated nuchal organ present dorsolateral to oral opening ( Fig. 51C inset); peristomium with three annular rings, first two rings merged dorsally, divided laterally, third ring largest, with pair of dorsal tentacles arising from near posterior margin ( Figs. 51B , 53 A–B); first pair of branchiae located posterior to tentacles, but anterior to setiger 1 ( Figs. 51B , 53 A–B). Second pair of branchiae on setiger 1, dorsal to notosetae on posterior margin, subsequent branchiae in similar location ( Figs. 51B , 53A ). In most specimens, dorsal tentacles and branchiae relatively short throughout body ( Figs. 51B , 52A ).

Parapodia reduced with setae emerging directly from body wall; in far posterior setigers parapodia formed into low membranes bearing partial cinctures of spines and capillaries ( Fig. 53D ). Noto- and neurosetae from setiger 1 all capillaries ( Fig. 53B , E–F), numbering 6–10 per fascicle; individual capillaries with numerous fibrils arising from shaft, best seen with SEM ( Fig. 53G ). Some specimens with long, natatory-like capillaries along most of the body. Acicular spines first occur from setigers 20–25 in neuropodia and setigers 25–30 in notopodia; in smaller specimens, these spines occur more anteriorly, but usually associated with enlarged mid-body segments. In most posterior setigers noto- and neuroacicular spines formed into partial cinctures with spines emerging from low elevated membranes ( Figs. 52B , 53D ); notopodial spines numbering 6–7 and neuropodial spines 5–6, or with 11–13 spines on a side; spines accompanied by closely situated, thin, smooth capillaries; spines broad, tapering to a pointed tip ( Figs. 51C , 52B , 53H ).

Pygidium simple rounded lobe ventral to anal opening ( Fig. 53D ).

Variability. Most of the variability observed in C. shackletoni n. sp. is likely due to contraction during preservation. The peristomial rings are more obvious in specimens where the pre-setiger region appears shortened, likely due to contraction. The presence of a mid-dorsal groove on the anterior thoracic segments is difficult to observe and may be an artifact of preservation. The pigmentation present along most of the body in preservative is either reddish or brown; this pigment is faded in some specimens or entirely absent. In most specimens, the dorsal tentacles and branchiae are missing, but when present, they are often short, not elongate. In the largest specimens, the swollen middle section is also longer and encompasses more segments. In general, the prostomium is short, not elongate as in C. biannulata n. sp.

Methyl Green stain. Body stains uniformly throughout, with no distinct pattern.

Etymology. This species in named in honor of Sir Ernst Shackleton (1874–1922), Antarctic explorer of the Heroic Age, whose Imperial Trans Antarctic Expedition (1914–1917) was curtailed by ice in the Weddell Sea, resulting in his ship the HMS Endurance being trapped, eventually crushed, and sunk, leaving all hands with few options for survival. Shackleton, however, organized the crew and after hauling their life boats across the ice to open water, they eventually landed on Elephant Island. There being no hope for rescue, Shackleton led a small crew on his now-famous boat journey to South Georgia where eventually, after a trek over the mountains from the southern to northern side of the island to the whaling station at Grytviken, he was able to find a vessel, return, and rescue his crew with no loss of life. Years later, while on another expedition, he died on 5 Jan 1922 on South Georgia where he was buried.

Remarks. Chaetozone shackletoni n. sp. is a deep-water species that closely resembles C. biannulata n. sp. with which it may co-occur. Chaetozone shackletoni n. sp. differs in having a shorter prostomium, a distinctly enlarged middle section along the body, brown to red pigmented areas along the body, and partial posterior cinctures of acicular spines with up to 13 spines on a side instead of up to 24 in C. biannulata n. sp. In life, the body of C. shackletoni n. sp. is pigmented green; in preservative, it is brown to red, but fades considerably after long periods of preservation.

The enlarged middle body segments and pigmentation of C. shackletoni n. sp. is reminiscent of another deepwater species, C. brunnea , from the eastern North Pacific ( Blake 2006). In C. brunnea , however, the enlarged area is limited to only a few segments, which actually produce a twist on the body resulting in the posterior segments often being offset from the anterior segments. In C. brunnea , the enlarged segments function as a stomach that is filled with sediment. In C. shackletoni n. sp., the enlarged segments are more posterior on the body, occupy up to 10 segments, and, based on observations, may simply be a site where eggs are stored and mature. An enlarged stomach also occurs in C. australosetosa n. sp. and C. biannulata n. sp. but is rarely conspicuous.

Habitat. Sediments in areas where Chaetozone shackletoni n. sp. were collected were generally olive-grey sandy muds, with silt content being the highest fraction. At the Weddell Sea station the sediment was moderately to poorly sorted: sand (9%), silt (72%), and clay (19%) ( Howe et al. 2007).

Distribution. Antarctic Seas: Drake Passage, Scotia Sea, and Weddell Sea in slope depths, 1622–2288 m.

Figures

FIGURE 51. A, Chaetozone reticulata n. sp. Holotype (USNM 46817), anterior end dorsal view. Arrows denote pigment. B–C, Chaetozone shackletoni n. sp. USCG Glacier Sta. 69-20 (USNM 1490776): B, anterior end, dorsal view; C, posterior neurosetae.

FIGURE 52. Chaetozone shackletoni n. sp. A, anterior two-thirds of worm photographed in life, left lateral view; B, posterior parapodium, left anterior view; C, entire worm photographed in life; D, anterior end, right lateral view. A, C–D, ANDEEP I, Sta. 105-5, paratypes (SMF 24936); B, USCG Glacier Sta. 69-20, paratype (USNM 1490776).

FIGURE 53. Chaetozone shackletoni n. sp. SEMs of paratype, ANDEEP III Sta. 78-6 (MCZ 149853): A, anterior dorsal view; B, anterior end, right lateral view; C, pre-setigerous region, right lateral view (inset of nuchal organ, not to scale); D, posterior end, left lateral view; E, peristomium and setiger 1, right lateral view; F, setigers 1–4, right lateral view; G, detail of capillary notosetae; H, posterior neuroacicular spines and capillaries.